1. Field of the Invention
This invention relates to a crystal growth method and apparatus.
2. Background Art
Metal-organic chemical vapor deposition (MOCVD) is used for crystal growth of compound semiconductors because of its good controllability on film thickness and composition. However, growth of nitride semiconductors such as GaN requires higher crystal growth temperature than that of AlGaAs-based and InAlGaP-based materials, hence causing various problems.
For example, in the so-called horizontal flow scheme where a wafer is held nearly horizontal and the flow direction of raw material gas and carrier gas is nearly parallel to the wafer, the gas is emitted from a nozzle located in the vicinity of the center of the crystal growth chamber. In this case, the gas temperature cannot be sufficiently decreased on the upstream side, resulting in thermal decomposition of the gas and its associated intermediate reaction. Consequently, the raw material element tends to run short in the vicinity of the wafer, and the semiconductor film growth rate often becomes lower than its maximum.
In particular, as a result of intermediate reaction associated with the decomposition of trimethylaluminum and ammonia, aluminum (Al) runs short on the downstream side. Thus, unfortunately, it is difficult to increase the Al composition ratio.
JP 2003-257867A discloses a technique for improving uniformity and controllability of crystal growth film across a wafer. This technique is directed to improving uniformity of electron mobility in AlGaAs-based or other materials by controlling the temperature of the region from the center of the susceptor to the wafer to be lower than that on the substrate. However, further improvement is needed for nitride materials because of their higher crystal growth temperature.